For power converters operating at powers larger than approximately 100 watts (W) at full load, a resonant topology provides a solutions with high efficiency and small volumes/high power density. At power levels above 100 watts, the extra cost compared to other topologies (e.g., an extra switch, extra secondary diode, resonant capacitor) is compensated for by additional advantages of the resonant topology. There are several types of resonant converters, such as a series resonant converter, an LLC converter, and an LCC converter. The series resonant converter uses a resonant capacitor, Cr, and an inductor, Ls, as resonating components while LLC and LCC converters use three resonant components. For an LLC converter, the magnetizing inductance of the transformer takes part in the resonance, while for an LCC converter, an extra capacitor, which takes part in the resonance, is present at the secondary side of the transformer.
Resonant power supplies are being used in light emitting diode (LED) television applications to provide a low voltage output of about 12V DC that supplies the low voltage circuits and a high voltage output of around 165V that supplies the LED strings for the backlight of the display. Such resonant power supplies typically include a main regulation loop that senses the 12V output and regulates the power level of the converter in order to keep the 12V output constant while the load varies. The 165V output then follows the 12V by a more or less fixed ratio as set by the turns ratio of the resonant transformer. Because the 165V output is not regulated, the output voltage of the 165V output can vary considerably with load variations on both the 165V output and the 12V output. Thus, a second control stage is often used after the 165V output to provide for a more accurate supply voltage for the LED strings. However, a second control stage adds cost to such resonant power supplies.